FieldDataLoader

DataLoader is a utility for batching and caching requests to backend services. FieldDataLoader is a lightweight wrapper that adds support for requesting specific fields per item. For example:

import { FieldDataLoader } from 'fielddataloader';

var userLoader = new FieldDataLoader((key, fields) => {
  return db.select(/* ... */);
});

// Then, from different places in your app:
userLoader.load(1, ['first_name']);
userLoader.load(1, ['last_name']);
userLoader.load(2, ['first_name']);

// These will be batched into a single callback per ID, for example:
//   SELECT first_name, last_name FROM users WHERE id = 1;
//   SELECT first_name FROM users WHERE id = 2;

This is especially handy for GraphQL resolvers! Use it with the included getSelection helper to load only the specific fields that were requested in the given GraphQL query from your database or backend. For example:

import { FieldDataLoader, getSelection } from 'fielddataloader';

const resolvers = {
  Query: {
    user(obj, args, context, info) => userLoader.load(args.id, getSelection(info)),
  }
}

Getting Started

First, install FieldDataLoader using npm.

npm install --save fielddataloader

To get started, create a FieldDataLoader. Each DataLoader instance represents a unique cache.

Usage

FieldDataLoader replicates most of DataLoader's API:

const dataLoader = new FieldDataLoader(batchLoadFn)

Create a new FieldDataLoader, given a batch loading function.

• batchLoadFn: A function which accepts a key and an array of fields, and returns a Promise which resolves to an Array of values.

dataLoader.load(key, fields)

Loads fields for the given key, returning a Promise for the result.

• key: A key value to load.
• fields: A list of fields to load for that key.

dataLoader.clear(key)

Clears the value at key from the cache, if it exists. Returns itself for method chaining.

• key: A key value to clear.

dataLoader.clearAll()

Clears the entire cache. To be used when some event results in unknown invalidations across this particular FieldDataLoader. Returns itself for method chaining.

getFields(info, [returnType], [path])

Given the info argument from a GraphQL resolver, returns an array containing the names of any requested fields. Supports named & inline fragments, and fields referenced by Apollo Federation's @requires directive. You can optionally pass a returnType and path if reading fields nested within a query (rather than at the root).

License

© DoSomething.org. FieldDataLoader is free software, and may be redistributed under the terms specified in the LICENSE file. The name and logo for DoSomething.org are trademarks of Do Something, Inc and may not be used without permission.